I actually agree with your first statement - I saw that a few days ago, and I'm keeping it in mind provisionally.

As to the second point, humans are not very genetically diverse. Of course there are enough differences that we can sometimes conclude that a particular person's genes come from an ethnic group, but it's not as nearly as varied as even other primates. Basically, we hit a genetic bottleneck sometime in the past 100,000 years. So it's far more likely to be a result of culture than genes.

You're right about the lack of genetic diversity, but even that limited amount of diversity can result in drastically different personalities, tendencies, etc. For some truly shocking material on just how much of a role genes can play, look into the studies done on identical twins raised apart vs adoptive siblings raised together.